Question

Stiind ca a=1+2+3…+2011 atunci a =?

Answer 1

a=1+2+3+.....+2011

a=(1+2011)*2011/2

a=2012 *2011/2

a=1006 *2011

a=2 023 066

Answer 2

$\frac{n(n+1)}{2} \\ n=ultimul.numar.din.sir \\ \\ \frac{2011(2011+1)}{2} = \frac{2011*2012}{2}=2011*1006=2023066$